Multilayer organic photovoltaic elements

ABSTRACT

A novel multilayer, organic composition, a photovoltaic element fabricated therefrom having enhanced conversion efficiencies, and their use to generate power, are disclosed. Compounds with generally planar polycyclic nuclei such as organic photoconductive dyes comprise the several layers of the composition.

RELATED APPLICATIONS

This application is a continuation-in-part application of U.S. Ser. No.821,115, filed on Aug. 2, 1977, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to photovoltaic elements useful for convertinglight and particularly for converting solar energy into electricalenergy. The invention features the use of organic compounds.

2. State of the Prior Art

So-called Schottky barrier or P-N junction photocells rely upon the factthat a built-in potential exists at the metal/semiconductor interface asin the Schottky device or at the junction between the P-type and N-typesemiconductors as in the P-N junction device. Electron-hole pairsgenerated by the absorption of light in the semiconductor are separateddue to the built-in field at the interface, establishing an electricalpotential.

Among chief materials used in the past for solar cells have beeninorganic semiconductors, due to their fairly high conversionefficiencies which have been as high as 12 to 15 percent, for example,for silicon. However, such devices have proven to be very expensive toconstruct, due to the melt and other processing techniques necessary tofabricate the semiconductor layer. As a result, such devices have hadextensive practical utility only in the field of space exploration, andnot in terrestrial applications.

In an effort to reduce the cost of solar cells, organic photoconductorsand semiconductors have been considered, due to their inexpensiveformation by solvent coating and similar techniques. However, prior artorganic materials have generally produced solar cells with conversionefficiencies only as high as about 0.05 percent at their highest, whenexposed to incident sunlight at an intensity of 100 mW/cm². An exampleof such a material is crystal violet, as described, for example, in U.S.Pat. No. 3,844,843. Still higher efficiencies at least as high as 0.1percent are desirable if the cells are to have practical terrestrialuse, notwithstanding their inexpensive cost of manufacture. Anefficiency of 0.3 percent was reported as being achieved through the useof an undisclosed dopant, as noted in "Prospects for Direct Conversionof Solar Energy to Electricity," AWA Technical Review, Volume 15, No. 4,1974, footnote 3, but none of the materials used has been disclosed.

Solar cells utilizing other organic photoconductive materials aredisclosed in U.S. Pat. Nos. 3,009,006; 3,057,947; 3,507,706; 3,530,007;and IBM Technical Disclosure Bulletin 18 (8), page 2442 (January 1976).However, there is no disclosure in any of these publications how tomanufacture a solar cell which exhibits a conversion efficiency highenough for extensive practical terrestrial use, i.e., greater than about0.1 percent.

Multilayer photoconductive compositions have been formulated in thepast, for xerographic application, using porphyrinic compoundsoverlayered with a charge-transport layer, as disclosed, for example, inU.S. Pat. Nos. 3,895,944 and 3,992,205. However, such charge-transportlayers in U.S. Pat. No. 3,895,944 have required the use of binders, aswell as sensitizers, and in U.S. Pat. No. 3,992,205 the layer containingphthalocyanine requires the use of another pigment admixed therewith.

Phthalocyanine, a porphyrinic compound, has been used in organic solarcells in the past, in contact with a layer of electron acceptors such asoxidized tetramethyl p-phenylenediamine, β-carotene, dibrominatedp-phenylenediamine, p-chloranil and the like. Examples are illustratedin U.S. Pat. No. 3,057,947. However, such cells have extremely lowconversion efficiencies, less than 10⁻⁷ percent (power output, col. 3,line 69, divided by 100 milliwatt input) for several reasons. First, theacceptors are not dyes and therefore do not absorb radiation in thevisible spectrum as well as dyes do. Second, the layers are formed bypressing techniques and, as such, require thicknesses which are far toolarge for efficient solar cells.

Multilayer photoelectric cells have been constructed from aphthalocyanine layer with or without an overcoat of malachite green, asreported, for example, in Topics in Current Chemistry, Springer-Verlag,Volume 61, 1976, page 124, and U.S. Pat. No. 3,789,216, issued Jan. 29,1974. However, the conversion efficiency of such cells were verylow--less than 10⁻⁴ percent, as reported in Springer-Verlag.

A layer of porphyrin or porphyrin-like material has also been used inthe past to improve already existing solar cell semiconductors, such asselenium. Examples are disclosed in U.S. Pat. No. 3,935,031. However,only expensive inorganic semiconductors which themselves areself-sufficient cell materials have been suggested for such use withporphyrin.

Pyrylium and thiapyrylium dyes have been disclosed for use assensitizers in photoconductive compositions, as noted, for example, inU.S. Pat. Nos. 3,938,994 and 3,997,342. No mention is made in thesepatents, however, as to the dye being useful with an adjacent layer ofporphyrinic compound.

Other patents relating to the general background of organic solar cellsinclude U.S. Pat. No. 3,912,931, issued Oct. 14, 1975.

Other patents relating to the general background of photoconductorcompositions having a charge generating layer and a separate layerincluding a charge transport compound include U.S. Pat. Nos. 3,591,374,issued July 6, 1971; 3,837,851, issued Sept. 24, 1974; 3,840,368, issuedOct. 8, 1974; 3,996,049, issued Dec. 7, 1976; and 3,955,978, issued May11, 1976.

RELATED APPLICATIONS

U.S. Pat. No. 4,125,414 based on commonly owned U.S. application Ser.No. 885,926, filed on Mar. 13, 1978, a continuation-in-part applicationof U.S. Ser. No. 821,117, now abandoned filed on Aug. 2, 1977 by C. W.Tang et al. entitled "Organic Photovoltaic Elements," discloses elementscomprising an organic photoconductive layer which includes pyrylium-typedyes together with a binder and a photoconductor. A preferred method ofmaking such a composition features the formation of a discretediscontinuous phase in a continuous phase. A very thin nucleating layerof copper phthalocyanine can also be used with this photoconductivelayer, but it does not form a rectifying junction.

OBJECTS OF THE INVENTION

It is an object of the invention to provide an organic solar cell usinga multilayer organic composition, such a cell having improved conversionefficiencies.

It is another object of the invention to provide certain novelmultilayer organic photoconductive laminates.

It is a related object of the invention to provide a solar cell which isboth inexpensive to produce and sufficiently efficient as to be usefulin terrestrial applications.

Other objects and advantages will become apparent upon reference to thefollowing Summary and Description of the Preferred Embodiments, whenread in light of the attached Drawing.

SUMMARY OF THE INVENTION

The invention concerns solar cells and an organic, multilayercomposition thereof, such solar cells exhibiting enhanced electricalresponse to incident light.

More specifically, there is provided an improved photovoltaic elementcomprising a first layer of an organic, electron donor material; asecond layer of an organic electron acceptor material in contact withsaid first layer; at least one of said materials being capable ofabsorbing light at wavelengths between about 350 and about 1000 nm andboth of said materials being capable of forming a rectifying junctionbetween them; and electrodes in operative ohmic contact with at leastpart of said layers. The improvement in the cells is that each of thematerials comprises a compound containing a generally planar polycyclicnucleus, and when laminated together, the layers produce a totalcombined thickness no greater than about 0.5 microns and a conversionefficiency for said element of at least about 0.02% when exposed to anAM2 light source.

In another aspect of the invention, each of the organic materials ofsuch a cell comprises a compound containing a generally planar,polycyclic nucleus having a surface area of at least about 40 squareangstroms and a width of at least about 5 angstroms, and the combinedthickness of said layers does not exceed about 0.5 microns.

The invention also concerns the use of such cells to generate electricpower from incident radiation.

In another aspect, the invention concerns also a multilayerphotoconductive laminate comprising a layer of an organic electron donorcompound containing a generally planar polycyclic nucleus, and incontact with said layer, a layer of a photoconductive organic dye havingthe structure: ##STR1## wherein: J is ##STR2## or N; Q and X are thesame or different and are each O, S, or Se;

R⁸, r⁹ and R¹⁰ are the same or different and are each H, alkyl from 1 toabout 3 carbon atoms, aryl, cyano or nitro;

R¹, r², r³ and R⁴ are the same or different and are each phenyl, oralkyl or alkoxy containing from 1 to about 5 carbon atoms, at least twoof R¹, R², R³ and R⁴ being phenyl;

m is 1 or 0;

and Z.sup.⊖ is an anion.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a sectional view, partly schematic, of a cell constructedin accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although the invention is hereinafter described in terms of itspreferred embodiment, photovoltaic elements or solar cells, it is notlimited thereto. Rather, the composition of the invention which makessuch solar cells possible can also be used in a photoconductive mode orenvironment other than solar cells, for example, in a photodiode or as aphotoconductive element in an electrophotographic imaging process. Insuch other environments, the thicknesses of the layers can vary fromthat desirable for solar cells, depending on the particular use. Also,only one electrode need be disposed in ohmic contact, in mostphotoconductive uses.

As used herein, "photovoltaic element" or "cell" means a solid statedevice which converts radiation absorbed by the element, directly toelectric power. A photovoltaic element of this invention is suitable asa terrestrial rooftop generator or as a light-level measuring device. Asa light-level measuring device, the cell can be used both at high andlow light levels. The cell exhibits a moderately high open circuitvoltage of about 300-500 mV.

Alternatively, the cell can also be used in the current mode. Thecurrent generated in a diffuse room-light condition is about 40 μA/cm²,a large enough current to be measured accurately. The current can thusbecome a measure of the light intensity, and the calibrated cell can beused as an exposure meter and find application in cameras.

In accordance with one aspect of the invention, organic solar cells areprovided with conversion efficiencies heretofore unattainable, that is,at least about 0.02% and as high as 1%. This is achieved by the use of alayer of an organic electron donor material in contact with a layer ofan organic electron acceptor material, each of which comprises acompound with a generally planar polycyclic nucleus. Together the layershave a combined thickness that does not exceed about 0.5 microns.

The terms "electron donor" and "electron acceptor" are used to describethe respective materials' electron affinity, particularly when thematerials are considered as a pair. Thus, an electron donor material,hereinafter "donor material," has a relative low electron affinity, andan electron acceptor, hereinafter "acceptor material," has a relativelyhigh electron affinity. As such, a donor material tends to act as ap-type material whereas an acceptor material tends to act as an n-typematerial. The two materials when layered together form a rectifyingjunction between them, and at least one of them is capable of absorbinglight at wavelengths between about 350 and about 1000 nm.

As used herein, a rectifying junction is one that provides a ratio offorward current to reverse current of at least about 10 when the elementor cell is biased by at least 0.5 v. "Polycyclic" is used in itsbroadest context to include both a plurality of unfused rings as well asa plurality of fused rings.

Any organic donor and acceptor material with a generally planar,polycyclic nucleus can comprise one of the two respective contactinglayers as long as such material has the afore-described characteristicsand permits the formation of a solar cell with a conversion efficiencyof at least 0.02% when exposed to simulated AM2 sunlight. The generalplanarity of the compounds' nuclei provides several desirablecharacteristics. First, it insures that cells made from coatings ofthese compounds will be generally free from shorts due to pinholes inthe coatings. That is, such compounds, when coated, deposit as flat,overlying molecules. Second, the planarity of the compounds' nucleiprovides minimum resistance to charge transport through the two layersof the compounds, and therefore provides a maximum I_(sc).

In one preferred embodiment of the invention, the generally planarpolycyclic nuclei of the compounds are highly conjugated withPi-electrons and have a large surface area. Generally, the larger themolecular surface area of such compounds, the more likely it is that thecompounds will provide a cell with high conversion efficiencies. It isthought that this greater area provides greater assurance of molecularoverlap in the layer formed, and less likelihood of pinhole shorts.Particularly useful examples of such compounds containing large-surfaceareas are those having a surface area in the plane of the compound onone side of the compound of at least 40 square angstroms and a width (inthe plane) of at least about 5 A.

As noted, one of the two contacting layers of the solar cell of theinvention comprises a material that is donor-like, and the other amaterial that is acceptor-like. Each layer can comprise just onecompound from that type only, or a mixture of that type only.

Examples of such donor compounds with generally planar, fused polycyclicnucleii include phthalocyanine and porphyrinic compounds. Any suchporphyrinic compound is operative, with or without metal, and any metalwill work, such as cobalt, magnesium, zinc, palladium, nickel, copper,lead, and platinum. However, some metal phthalocyanines are preferredbecause of the greater conversion efficiencies which they exhibit.Examples of preferred metal phthalocyanines include copper, lead, andplatinum phthalocyanine. For example, lead phthalocyanine has produced acell with a spectral response extending to almost 1000 nm. Further, itis preferred that the porphyrinic layer be noncrystalline, as thepresence of crystals tends to provide a shorting path which can renderthe element inefficient.

As used herein, a porphyrinic compound is any compound, material orsynthetic which derives from or includes the basic porphyrin structure.Examples of such are disclosed in the aforesaid U.S. Pat. No. 3,935,031,the details of which are expressly incorporated herein by reference. Acurrently preferred class of such compounds is the class having thestructure: ##STR3## wherein: M is a metal;

T¹ and T² are both S or both C, or one of T¹ and T² is N and the otherC;

X¹ and X² are the same or different, and are each hydrogen or halogen,such as chlorine, fluorine, bromine and the like; and

Z¹ is the nuclear carbon atoms necessary to form a six-memberedunsaturated ring.

A further option is to provide the compounds with the structure of (I),but in a nonmetallic complex, whereby two of the four nitrogens becomehydrogenated.

It has been discovered further that the layer of donor materials can bedivided into two contiguous layers of different phthalocyanines only oneof which contacts the layer of acceptor material, the other of which, inthe case of a solar cell, is in ohmic contact with the electrode for thephthalocyanine. In such a case, the total thickness of the two layers ofphthalocyanine considered together should equal the thickness that wouldbe used for a single phthalocyanine layer.

Still other examples of useful donor compounds with generally planar,fused polycyclic nuclei include those containing at least 8 carbocyclicfused rings. Examples include ovalene,diindeno[1,2,3-cd-1',2',3'-lm]perylene, violanthrene, isoviolanthrene,and pyranthrene.

Examples of useful acceptor compounds with generally planar, fusedpolycyclic nuclei include those having the structure: ##STR4## whereinZ² and Z³ together comprise from about 12 to about 32 nonmetallic atomsnecessary to complete between about 5 and about 10 fused aromatic orheterocyclic rings,

Z⁴ and Z⁵ are the same or different, and each comprise from 2 to 8nonmetallic atoms necessary to complete 1 or 2 fused aromatic orheterocyclic rings,

R¹ through R⁶, and G¹ and G² are the same or different and are each H;an electron-withdrawing group such as keto, cyano, halide such aschloride, bromide and the like, sulfonyl, carboxy, nitro and the like;imino; alkyl or alkoxy containing from 1 to 5 carbon atoms, for example,methyl, ethyl, propyl, hydroxyl; amino; aryl containing from 6 to 10carbon ring atoms, e.g., phenyl, naphthyl; or together, R¹ and R² or R⁵and R⁶ can comprise 4-8 nonmetallic atoms necessary to complete one ortwo aromatic rings; provided that at least one of R¹ through R⁶ and G¹and G² is an electron-withdrawing group. As used herein, "alkyl" and"aryl" include substituted alkyl, such as hydroxypropyl, methoxy,ethoxy, butoxy, and the like; and substituted aryl, respectively, suchas phenol, halophenyl, alkoxyphenyl, alkylphenyl, and the like.

Representative examples of fused polycyclic compounds having thestructure noted are anthraquinone vat pigments such as flavanthrone,perylene derivatives, coronone-imide derivatives, ovalene derivatives,and compounds of the structure ##STR5## wherein E is O or S.

For example, the following perylene derivatives are particularly useful:##STR6## wherein R⁷ and R⁸ are the same or different and are eachhydrogen, alkyl containing from 1 to 5 carbon atoms, such as methyl,ethyl, propyl, hydroxypropyl and the like; phenyl such asp-chlorophenyl, p-alkoxyphenyl, p-methylphenyl and the like; orquinolyl; and R⁹, R¹⁰, R¹¹, and R¹² are each ═O; or R⁷ and one of R⁹ andR¹⁰, and R⁸ and one of R¹¹ and R¹² together comprise from 7 to 8nonmetallic atoms necessary to form one or two fused carbocyclic orheterocyclic rings, in which case the other of R⁹ and R¹⁰ and the otherof R¹¹ and R¹² is ═O. For example, included in A are ##STR7##

Other useful examples of acceptor materials include those in which thepolycyclic rings are not fused. Particularly useful examples arephotoconductive organic dyes such as pyrylium-type dye salts whichinclude pyrylium, thiapyrylium and selenapyrylium dye salts, and alsosalts of the aforementioned pyrylium-type dye salts containing condensedring systems such as salts of benzopyrylium and naphthopyrylium dyes.Highly preferred examples having a generally planar nucleus, a surfacearea of at least 40 square angstroms and a width in the plane of atleast 5 A, are those with the structure: ##STR8## wherein: J is ##STR9##or N; Q and X' are the same or different and are each O, S or Se;

R¹⁷, r¹⁸ and R¹⁹ are the same or different and are each hydrogen; alkylfrom 1 to about 3 carbon atoms such as methyl, ethyl, iso-propyl and thelike; aryl such as phenyl and naphthyl and including substituted aryl;cyano or nitro;

R¹³, r¹⁴, r¹⁵, and R¹⁶ are the same or different and are each phenyl,including substituted phenyl, or alkyl or alkoxy containing from 1 toabout 5 carbon atoms, such as methyl, ethyl, iso-propyl, methoxy,propoxy and the like, at least two of R¹³, R¹⁴, R¹⁵ and R¹⁶ beingphenyl;

m is 1 or 0 and is 0 if J is N;

and Z⁷⊖ is an anionic moiety, such as perchlorate, fluoroborate, and thelike.

If R¹³, R¹⁴, R¹⁵ and R¹⁶ are substituted phenyl, it is preferred thatthe substituents be located in the para position and be selected fromamong those which shift the blue absorption peak of the dye to a longerwavelength. Useful examples of such substituents include alkyl from 1 to3 carbon atoms and halogens such as chlorine, fluorine and the like.

Dyes of structure (II) above can be manufactured by any convenientmethod. For example, the process disclosed in Helvetica Chemica Acta,Volume 49, Fasciculus 7, 1966, No. 244, pages 2046 through 2049 can beused.

It is contemplated that another class of useful acceptor polycycliccompounds of the unfused type is 2,4,6-trisubstituted pyrylium andthiapyrylium dye salts of the general structure: ##STR10## in which R²⁰and R²¹ are the same or different and are each alkyl from 1 to about 6carbon atoms, such as methyl, ethyl, iso-propyl and the like; phenyl,including substituted phenyl; or a 5 or 6 membered heterocyclic ring,such as thienyl, furyl, pyridyl, pyrimidinyl, thiadiazolyl or thiazolylor pyrrolyl; R²² represents an alkylamino-substituted phenyl or analkylamino-substituted 5 or 6 membered heterocyclic ring having from 1to about 6 carbon atoms in the alkyl moiety includingdialkylamino-substituted and halogenated alkylamino-substituted phenyldialkylaminopyridyl, dialkylaminofuryl, dialkylaminothienyl,dialkylaminopyrimidinyl, dialkylaminothiadiazolyl ordialkylaminothiazolyl; X is oxygen, selenium or sulfur and Z⁸⊖ is ananion such as perchlorate, fluoroborate, and the like. Examples of suchcompounds, particularly wherein at least one of R²⁰, R²¹ and R²² isheterocyclic, are described and claimed in commonly owned U.S.application Ser. No. 711,046, filed Aug. 2, 1976, by D. P. Specht et al,entitled "Sensitizers for Photoconductive Compositions," and in ResearchDisclosure, Volume 157, May 1977, Publication No. 15742, published byIndustrial Opportunities, Limited, Homewell, Havant, Hampshire, PO9 1EF,United Kingdom, the details of which are expressly incorporated hereinby reference.

Yet another useful class of pyrylium type dyes are disclosed incommonly-owned U.S. application Ser. No. 813,371 filed on July 6, 1977,entitled "Novel Radiation Sensitive Compounds and Radiation SensitiveCompositions Containing the Same" by M. Petropoulos et al.

Representative useful dyes having structures of the type (II) or (III)described above include:

4-[(2,6-diphenyl-4H-thiapyran-4-ylidene)methyl]-2,6-diphenylthiapyryliumperchlorate,

4-[(2,6-dimethoxy-4H-thiapyran-4-ylidene)methyl]-2,6-diphenylthiapyryliumperchlorate,

4-[(2,6-diphenyl-4H-pyran-4-ylidene)methyl]-2,6-diphenylthiapyryliumperchlorate,

4-[(2,6-diphenyl-4H-pyran-4-ylidene)methyl]-2,6-diphenylpyryliumfluoroborate,

4-[(2,6-diphenyl-4H-thiapyran-4-ylidene)methyl]-2,6-diphenylselenapyryliumperchlorate,

4-[(2,6-diphenyl-4H-selenin-4-ylidene)methyl]-2,6-diphenylselenapyryliumperchlorate,

4-[(2,6-diphenyl-4H-pyran-4-ylidene)methyl]-2,6-diphenylselenapyryliumperchlorate,

4-[(2,6-diethyl-4H-thiapyran-4-ylidene)methyl]-2,6-diphenylthiapyryliumperchlorate,

4-[(2,6-diphenyl-4H-thiapyran-4-ylidene)methyl]-2,6-diethoxythiapyryliumperchlorate,

2,6-diphenyl-4-[(2,6-diphenyl-4H-pyranylidene)amino]pyryliumperchlorate,

2,6-diphenyl-4-(4-dimethylaminophenyl)thiapyrylium hexafluorophosphate,

2,6-diphenyl-4-(4-diphenylaminophenyl)thiapyrylium perchlorate,

2,6-diphenyl-4-(4-dipropylaminophenyl)thiapyrylium perchlorate,

4-{[2,6-di(p-methylphenyl)-4H-thiapyran-4-ylidene]methyl}-2,6-diphenylthiapyryliumperchlorate,

4-{[2,6-di(p-fluorophenyl)-4H-thiapyran-4-ylidene]methyl}-2,6-diphenylthiapyryliumperchlorate,

4-{[2,6-di(p-fluorophenyl)-4H-thiapyran-4ylidene]methyl}-2,6-di(p-fluorophenyl)thiapyryliumperchlorate,

4-{[2,6-di(p-methylphenyl)-4H-thiapyran-4-ylidene]methyl}-2,6-di(p-methylphenyl)thiapyryliumperchlorate.

The layer of acceptor material can comprise a mixture of two differentdyes of structure (II), or a dye of structure (II) with a dye ofstructure (III), in contact with the donor material layer. In fact, insome instances synergism has been demonstrated in that the conversionefficiency of the mixture exceeds that obtainable from using either oneof the dyes above, presumably because the dyes complement each other inthe mixture.

Two layers of different or the same acceptors also may be used in placeof a single layer of acceptor. For example, by sequential evaporation,two compounds of structure A can be used to provide two distinct layersof acceptors, one layer of which contacts a porphorinic donor layer toform a multilayer cell.

In addition to the materials described, other materials, compounds,compositions, and the like can be added, provided that they do notsignificantly decrease the conversion efficiency of the cell, preventthe layer from forming a rectifying contact with the layer of donormaterial, or create sinks for the light-generated carriers.

The thickness of the total composition comprising the combined layers ofdonor and acceptor materials is an important aspect of the photovoltaicelements of the invention, at least if maximum conversion efficienciesare desired. It has been found that efficiencies begin to decreasedrastically for a thickness in excess of about 0.5 microns, apparentlydue to failure of the light to penetrate to the region adjacent therectifying junction or to increased series resistance. Minimum thicknessfor the individual layers appears to be dictated more by coatingtechniques and the minimum that can be used without shorting. Usefuldevices of improved efficiency have been constructed with thicknessesfor each of the two layers as low as about 100 A.

Greater thicknesses can be used for other photoconductive uses orembodiments.

Currently preferred thicknesses for each of the two layers, for optimumsolar cell results, are about 300 to 500 A. If unequal thicknesses areto be used, it is preferred that the thinner layer be adjacent thewindow electrode, described hereinafter, to permit proper illuminationof the rectifying junction.

For a solar cell or photovoltaic element of the type described,electrodes are operatively connected, one to the donor layer and theother to the acceptor layer. Although the most common, and preferred,construction is one in which the electrodes are in physical contact withtheir respective layers, this need not always be the case. For example,the donor layer which contributes to the formation of the rectifyingjunction can be spaced away from its electrode by a layer of differentcompound as described above for the porphyrinic materials.

The electrodes for the cell are each selected to form an operative ohmiccontact with an adjacent layer. As used herein, "operative" meansconnected in a manner that does not short the cell and, as isconventional, is of a material which permits at least one of theelectrodes to be a window electrode. That is, at least one of theelectrodes is semitransparent to useful light. "Ohmic contact" means alow impedance contact with the adjacent layer, of no more than about1000 ohms/cm² impedance. Because the interface between the organicmaterials provides the necessary rectifying junction, the electrodematerials are selected to provide ohmic contacts as defined above. Theelectrode adjacent to the donor layer preferably has a high workfunction, while the one adjacent to the acceptor layer preferably has alow work function.

It has been found that a useful material for the electrode adjacent tothe donor layer is a glass or a transparent film such as poly(ethyleneterephthalate) coated with a semitransparent layer of indium tin oxide,tin oxide, or nickel. This material not only has a high work function,but its semitransparency makes it highly useful as the window electrode.Examples of such materials having a glass support are Nesa® andNesatron® glass manufactured by PPG Industries and having a sheetresistance of about 10 to 50 ohms/square and an optical transmittance ofabout 80 percent, for visible light.

The opposite electrode can be metal with a low work function, such asindium, silver, tin, aluminum or the like, and can be semitransparent oropaque. Silver is a preferred electrode for minimum loss in conversionefficiency upon aging.

As shown in the FIGURE, such a photovoltaic cell as described abovecomprises a laminar array 10 of a window electrode 12 comprising atransparent support 14 and a semitransparent layer 16 of indium tinoxide, tin oxide or nickel; a layer 18 of a donor compound, such asphthalocyanine that is metal free or metallic; a layer 20 of an acceptorcompound; and an electrode 22 of a metal having a sufficient workfunction as to form an ohmic contact with layer 20. It will beappreciated that the dimensions of the FIGURE have been exaggerated forclarity. Preferred thicknesses for the layers comprise, for layer 16,0.5 microns to about 5 microns; for layer 18, 100 to 2500 A; for layer20, 100 to 2500 A; and for electrode 22, 100 to 2,000 A. As noted above,in preferred constructions the combined thicknesses of layers 18 and 20do not exceed about 0.5 micron, for maximum efficiencies.

Wires 24 represent leads contacting the electrodes to connect the cellto a load circuit, as is conventional.

Such photovoltaic elements constructed from the materials describedabove have been found to produce markedly superior conversionefficiencies, at least as much as about 0.02 percent when exposed tosunlight, and even as high as 1.0 percent.

Any suitable coating technique can be used to manufacture thephotoconductive laminate and/or solar cell. For example, any coatingtechnique can be used wherein the two layers forming the rectifyingjunction are coated from two different solvents, one upon the other, thesolvent of one being a poor solvent for the other. In this manner, awell-defined interface between the two layers will be maintained. Analternative and highly preferred method is to vapor deposit theporphyrinic layer on a clean, i.e., polished, window electrode, usingsources of porphyrinic compounds which are reasonably free ofdecomposable or volatile materials, and thereafter solvent coat the dyelayer as by spin coating at between about 1,000 and about 10,000 rpmfrom one or more of the following solvents: 1,2-dichloroethane,dichloromethane, and mixtures of the two. For pyrylium dyes, aparticularly useful solvent mixture has been, by weight percent, 49%1,2-dichloroethane, 49% dichloromethane, and 2 percent1,1,1,3,3,3-hexafluoroisopropyl alcohol. Another preferred method is todeposit the two layers forming the rectifying junction by vapordeposition. This method is particularly useful for the polycyclic,perylene-type derivative acceptors. A currently preferred process forpolishing the Nesatron® glass, if used as the window electrode,comprises rubbing the Nesatron® surface with a cotton flannel wettedwith a suspension of an alumina or other abrasive, or by polishing in aspinning disc, usually for a few minutes. The polished Nesatron® glasswas then sonicated in a 1:1 H₂ O/isopropyl alcohol bath for about halfan hour to remove the abrasive particles, and then rinsed thoroughlywith distilled water. The polished Nesatron® glass appears relativelyclean in a strong light.

The electrode for the dye layer can be applied by conventional vapordeposition.

EXAMPLES

The following examples further illustrate the nature of the invention.In each case, a modified Kodak 600 slide projector, together withappropriate glass filters and a water filter, was used to provide asimulated AM2 sunlight, as defined in H. J. Hovel, "Solar Cells," 1975.The light incident on the cell had an intensity of 75 mW/cm² which wascalibrated against a standard silicon cell having a short-circuitcurrent output of 21.5 mA/cm² at 75 mW/cm², AM2. The current-voltagecharacteristics of each cell were traced by applying an external voltageto the cell in either polarity. The voltage across the cell and thecurrent through it (measured by a Keithley 602 multimeter) weresimultaneously traced by an x-y recorder.

EXAMPLE 1

A cell as shown in the FIGURE was fabricated in the following manner:

(a) A piece of Nesatron® glass about 1 inch by 1 inch was polished andthoroughly cleaned.

(b) A 400 A thick copper-phthalocyanine film was deposited on theNesatron® glass by vapor sublimation in a 1×10⁻⁵ torr vacuum.

(c) A 400 A thick layer of a photoconductive dye,4-[(2,6-diphenyl-4H-thiapyran-4-ylidene)methyl]-2,6-diphenylthiapyryliumperchlorate, was spin-coated on top of the copper phthalocyanine layer.This dye film appears to be homogeneous and very uniform.

(d) The top electrode, indium, was evaporated on top of the two-layerorganic component to complete the cell structure.

Under the simulated AM2 illumination described above (75 mW/cm²), thecell developed an open-circuit voltage of 0.36 volt, a short-circuitcurrent of 2 mA/cm², and a fill factor of 0.47. The power conversionefficiency was 0.45 percent.

EXAMPLES 2 THROUGH 7

A class of dyes having the formula: ##STR11## wherein φ is phenyl, and Xand Q are any two of the three elements O, S, Se, was tested as thephotoconductive material in a solar cell prepared as described forExample 1. The Cu-phthalocyanine layer deposited by vapor sublimationwas about 300 to 500 A thick, and the dye layer deposited on theCu-phthalocyanine by spin-coating was also about 300 to 500 A. Table Ilists the photovoltaic output of the cells when tested under simulatedAir-Mass-2 illumination.

                  TABLE I                                                         ______________________________________                                        Dye                           Conversion                                      Example Q     X     Voc (volts)                                                                            Isc (mA/cm.sup.2)                                                                      Efficiency, %                           ______________________________________                                        2       0     0     0.55     0.9      0.29                                    3       0     S     0.45     1.6      0.43                                    4       S     S     0.36     2.0      0.45                                    5       S     Se    0.31     1.5      0.28                                    6       Se    Se    0.24     2.0      0.28                                    7       0     Se    0.42     0.4      0.10                                    ______________________________________                                    

EXAMPLES 8 THROUGH 11

A class of dyes having the formula: ##STR12## wherein R¹² and R¹³ arepara substituents selected from H, CH₃, F, were tested as described inExample 1 in a photovoltaic cell of the configuration described inExample 1. Table II lists the output of these cells.

                  TABLE II                                                        ______________________________________                                                            Voc     Isc     Conversion                                Example                                                                                Dye        (volts) (mA/cm.sup.2)                                                                         Efficiency, %                             ______________________________________                                         8     R.sup.12 ═H,R.sup.13 ═CH.sub.3                                                    0.33     1.75    0.31                                       9     R.sup.12 ═H,R.sup.13 ═F                                                           0.30     2.15    0.35                                      10     R.sup.12 ═R.sup.13 ═F                                                             0.20     1.6     0.17                                      11     R.sup.12 ═R.sup.13 ═CH.sub.3                                                      0.40     1.3     0.28                                      ______________________________________                                    

EXAMPLES 12 THROUGH 19

Metal-free phthalocyanine and a number of metal-phthalocyanines wereused in a photovoltaic cell of the configuration described in Example 1.Phthalocyanine films of thickness ranging from 300 to 500 A weredeposited on clean Nesatron® glass, and a 400 to 500 A thick film of4-[(2,6-diphenyl-4H-thiapyran-4-ylidene)methyl]-2,6-diphenylthiapyryliumperchlorate was deposited on top of each phthalocyanine film byspin-coating. Indium was used as the top electrode. Table III lists theoutput of these cells comprising various phthalocyanine layers.

                  TABLE III                                                       ______________________________________                                                           Voc     Isc      Conversion                                Example                                                                              Phthalocyanine                                                                            (volts) (mA/cm.sup.2)                                                                          Efficiency, %                             ______________________________________                                        12     Metal-free  0.25    0.18     0.02                                      13     Co          0.20    0.35     0.03                                      14     Ni          0.25    0.80     0.09                                      15     Cu          0.36    2.00     0.45                                      16     Zn          0.36    1.10     0.14                                      17     Pb          0.35    3.50     0.50                                      18     Pd          0.42    0.75     0.14                                      19     Pt          0.38    1.25     0.21                                      ______________________________________                                    

EXAMPLE 20

A mixture of photoconductive dyes was used in the photovoltaic cell ofthe configuration described in Example 1. A 400 to 500 A thickCu-phthalocyanine film was deposited on clean Nesatron® glass by vaporsublimation. Then a 400 to 500 A thick film, containing a 1:1 mixture of4-[(2,6-diphenyl-4H-pyran-4-ylidene)methyl]-2,6-diphenylpyryliumperchlorate and4-[(2,6-diphenyl-4H-thiapyran-4-ylidene)methyl]-2,6-diphenylthiapyryliumperchlorate, was spin-coated on top of the Cu-phthalocyanine layer.Indium was the top electrode. Under simulated Air-Mass-2 illumination,the cell developed an open-circuit voltage of 0.43 volt, a short-circuitcurrent of 2 mA/cm², and a fill factor of 0.44, giving an efficiency of0.5 percent.

EXAMPLE 21

Example 20 was repeated, except that the mixture of dyes comprised a 1:1mixture of4-[(2,6-diphenyl-4H-thiapyran-4-ylidene)methyl]-2,6-diphenylthiapyryliumperchlorate and 2,6-diphenyl-4-(4-dimethylaminophenyl)thiapyryliumperchlorate. The conversion efficiency was found to be about 0.5percent.

EXAMPLE 22

In a cell configuration as described in Example 1, Nesatron®glass/copper-phthalocyanine (400 A/photoconductive dye/Ag, where thephotoconductive dye was a 400 to 500 A film of4-[(2,6-diphenyl-4H-thiapyran-4-ylidene)methyl]-2,6-diphenylthiapyryliumperchlorate and where silver was used as the top electrode, the celldeveloped an open-circuit voltage of 0.38 volt, a short-circuit currentof 1.8 mA/cm² and a fill factor of 0.4, giving a conversion efficiencyof 0.36 percent. The cell was quite stable under prolonged illumination.On subjecting the cell to a 90-hour exposure to simulated Air-Mass-2illumination, the cell reached a steady state efficiency of 0.23 to 0.25percent, with no evidence of further degradation.

EXAMPLE 23

A cell was fabricated as described for Example 1, but the dye used wasthe following: ##STR13##

The cell had a Voc=0.500 V, Isc=0.2 mA/cm², a fill factor of 0.28, andan efficiency of 0.05 percent.

EXAMPLE 24

Example 1 was repeated, except that the dye used was the following:##STR14## The cell had a Voc of about 0.5 V, an Isc of about 0.24mA/cm², a fill factor of about 0.34, and a conversion efficiency ofabout 0.5 percent.

EXAMPLES 25 THROUGH 27

Cells were fabricated as described for Example 1, except that dyes ofthe following structure were used: ##STR15##

Table IV lists the output of these cells for various substitutions at Q,X, and R.

                                      TABLE IV                                    __________________________________________________________________________                                            Conversion                            Example                                                                            Q X R               Voc (volts)                                                                          Isc (mA/cm.sup.2)                                                                     Efficiency, %                         __________________________________________________________________________    25   S S CH.sub.3        0.25   0.4     0.036                                 26   O O CN              0.4    0.4     0.066                                 27   O O                                                                                ##STR16##      0.44   0.28    0.05                                  __________________________________________________________________________

example 28

a photovoltaic element was prepared and tested as described in Example1, except that the dye layer, at a thickness of about 400 A, wascomprised of: ##STR17## This element was found to have a Voc of about0.52 V, an Isc of about 1 mA/cm², and a fill factor of 0.40, producing aconversion efficiency of about 0.27 percent.

EXAMPLE 29-36

A photovoltaic element was prepared and illuminated for each of theseexamples in the manner described for Example 1, except that acceptorcompounds of the structure ##STR18## wherein R⁷ and R⁸ were asdesignated in Table V, were used in place of the photoconductive dye ofExample 1, and a silver electrode was used in place of indium. Table Vsets forth the resulting solar cell properties.

                                      Table V                                     __________________________________________________________________________                                  Conversion                                                      Voc.                                                                              Isc   Fill                                                                              Efficiency                                      Example                                                                            R.sup.7R.sup.8                                                                           (mV)                                                                              (mA/cm.sup.2)                                                                       Factor                                                                            (%)                                             __________________________________________________________________________    29   CH.sub.3   385 1.93  0.55                                                                              0.55                                            30                                                                                  ##STR19## 440 3.0   0.6 1.0                                             31                                                                                  ##STR20## 330 1.5   0.48                                                                              0.32                                            32                                                                                  ##STR21## 400 1.4   0.52                                                                              0.4                                             33   H          330 1.65  0.50                                                                              0.36                                            34                                                                                  ##STR22## 280 1.1   0.44                                                                              0.2                                             35                                                                                  ##STR23## 530 0.5   0.31                                                                              0.1                                             36   CH.sub.2 CH.sub.2 CH.sub.2OH                                                             420 1.34  0.45                                                                              0.34                                            __________________________________________________________________________

examples 37-40

cells were prepared and illuminated as described in Examples 29-36,except that the acceptor material was the compound identified in TableVI for each respective example. The results are given in Table VI.

                                      Table VI                                    __________________________________________________________________________                                                            Conversion                                                    Voc  Isc   Fill Efficiency            Example                                                                            Compound                           (mV) (mA/cm.sup.2)                                                                       Factor                                                                             (%)                   __________________________________________________________________________    37                                                                                  ##STR24##                         490  1.7   0.57 0.65                  38                                                                                  ##STR25##                         480  0.8   0.17 0.1                   39                                                                                  ##STR26##                         530  1.00  0.34 0.24                  40                                                                                  ##STR27##                         450  1.7   0.61 0.66                  __________________________________________________________________________

in certain of the above examples, the cells so prepared were initiallyshorted, i.e., the Ag electrode was in contact with the Nesatronelectrode through pinholes in the organic layer. These cells were andcan be "recovered" simply by applying a large transient current throughthe cell which effectively burns out the shorts. This was done byshorting the cell momentarily with a 22-volt battery.

EXAMPLE 41

For this example, a cell as described in Example 1 was prepared andilluminated, except that in place of phthalocyanine, ovalene was used,Ag was used in place of indium, and each of the donor and acceptorlayers was about 500 A thick. The resulting cell had a conversionefficiency of about 0.1%.

EXAMPLE 42

A cell was prepared and illuminated as described in Example 41, exceptthat the donor material was diindeno[1,2,3,cd-1,2,3,lm]perylene and theacceptor material was flavanthrone. The resulting cell had a Voc=825 mV,Isc=0.8 mA/cm² fill factor=0.51, and a conversion efficiency=0.46%.

COMPARATIVE EXAMPLES C.E. No. 1: Inadequate Donor-Like Compounds

Coronene, a fused seven-ring carbocyclic compound, was used as theelectron donor material in a cell prepared as described in Example 1 inplace of the porphyrinic compound. The resulting cell was inoperativedue to permanent shorts that were present through the coronene layer.The cell did not produce a measurable conversion efficiency.

C.E. Nos. 2-7

Solar cells were prepared and illuminated as described in Example 1,with the exceptions noted hereafter, and in place of the acceptor dyetherein described, certain materials listed in Table VII were used toform the layer contacting the copper phthalocyanine.

                                      Table VII                                   __________________________________________________________________________                                                Conversion                                                    Voc  Isc    Fill                                                                              Efficiency                        C.E.                                                                             Compound                 (mV) (mA/cm.sup.2)                                                                        Factor                                                                            (%)                               __________________________________________________________________________    2  Malachite green*         550  0.1    0.22                                                                              0.015                             3  Crystal violet*          10-15                                                                              0.04-0.05                                                                            0.25                                                                              5 × 10.sup.-4               4  Rhodamine B*             680  0.055  0.37                                                                              0.018                             5  tetraphenyl                                                                   p-phenylenediamine*      4    0.01       negligible                            ##STR28##               190  0.07   0.25                                                                              4 × 10.sup.-3               7                                                                                 ##STR29##               150  0.05   0.25                                                                              2.5 × 10.sup.-3             __________________________________________________________________________     *For these examples, Ag was used in place of the indium electrode.       

The copper phthalocyanine/tetraphenyl p-phenylenediamine two layersystem yielded almost zero output. The cell was not shorted; rather theinternal resistance was on the order of 10⁶ ohm. It is expected thattetramethyl p-phenylenediamine (air oxidized) would behave similarly tothe tetramethyl derivative.

C.E. Nos. 8-10

Cells were prepared as described in C.E. Nos. 2-7, except thatanthraquinone, and different anthraquinone derivatives containing 4 or 6fused rings, respectively, were used as electron acceptor compounds. Inall cases, the cells were completely shorted. These examplesdemonstrated the preference, in the case of fused electron acceptorcompounds, for at least 7 aromatic rings in the nucleus. Compounds with6 or less rings have a surface area, measured in a plane, of less thanabout 40 square angstroms.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

What is claimed is:
 1. In a photovoltaic element including a first layer of an organic electron donor material, a second layer of an organic electron acceptor material in contact with said first layer, at least one of said mateials being capable of absorbing light at wavelengths between about 350 and about 1000 nm and both of said materials forming a rectifying junction between them, and electrodes in operative ohmic contact with at least part of said layers,the improvement wherein said materials each comprise a compound containing a generally planar polycyclic nucleus, and when laminated together, said layers produce a total combined thickness no greater than about 0.5 micron and a conversion efficiency for said element of at least about 0.02% when exposed to an AM2 light source.
 2. An element as defined in claim 1, wherein said polycyclic acceptor compound has as least 7 fused aromatic rings and said polycyclic donor compound has at least 8 fused aromatic rings.
 3. An element as defined in claim 1, wherein said acceptor compound is a dye or pigment.
 4. An element as defined in claim 1, wherein said acceptor compound is a dye selected from the group consisting of pyrylium, thiapyrylium and selenapyrylium dyes.
 5. In a photovoltaic element includinga layer of a compound having the structure: ##STR30## wherein: M is a metal;T¹ and T² are both S or both C, or one of T¹ and T² is N and the other C; X¹ and X² are the same or different, and are each halogen or hydrogen; and Z¹ is the nuclear carbon atoms necessary to form a six-membered unsaturated ring; an organic electron acceptor material capable of forming a rectifying junction with said compound, and electrodes in operative ohmic contact with at least part of said layers; the improvement wherein said acceptor material comprises a compound containing a generally planar polycyclic nucleus, and said layers when laminated together produce a total combined thickness no greater than about 0.5 microns and a conversion efficiency for said element of at least about 0.02% when exposed to an AM2 light source.
 6. An element as defined in claim 5, wherein said acceptor material has the structural formula: ##STR31## wherein R⁷ and R⁸ are the same or different and are each hydrogen, alkyl containing from 1 to 5 carbon atoms, phenyl, or quinolyl, and R⁹, R¹⁰, R¹¹, and R¹² are each ═O; or R⁷ and one of R⁹ and R¹⁰, and R⁸ and one of R¹¹ and R¹² together comprise from 7 to 8 nonmetallic atoms necessary to form one or two fused carbocyclic or heterocyclic rings, and the other of R⁹ and R¹⁰ and the other of R¹¹ and R¹² is ═O.
 7. An element as defined in claim 5, wherein said acceptor material includes a compound having a structural formula selected from ##STR32##
 8. An element as defined in claim 5, wherein said acceptor material has the structural formula ##STR33## wherein: J is ##STR34## or N; Q and X' are the same or different and are each O, S or Se;R¹⁷, r¹⁸ and R¹⁹ are the same or different and are each H, alkyl from 1 to about 3 carbon atoms, aryl, cyano or nitro; R¹³, r¹⁴, r¹⁵ and R¹⁶ are the same or different and are each phenyl, or alkyl or alkoxy containing from 1 to about 5 carbon atoms, at least two of R¹³, R¹⁴, R¹⁵ and R¹⁶ being phenyl; m is 1 or 0; and Z⁷⊖ is an anion.
 9. In a laminated photovoltaic element including a first layer of an organic electron donor material, a second layer of an organic electron acceptor material in contact with said first layer, at least one of said materials being capable of absorbing light at wavelengths between about 350 and about 1000 nm and both of said materials being capable of forming a rectifying junction between them, and electrodes in operative ohmic contact with at least part of said layers;the improvement wherein the combined thickness of said layers does not exceed about 0.5 micron and each of said materials comprises a compound containing a generally planar, polycyclic nucleus having a surface area of at least about 40 square angstroms and a width of at least about 5 angstroms, whereby said element has a conversion efficiency of at least about 0.02% when exposed to an AM2 light source.
 10. An element as defined in claim 9, wherein said polycyclic acceptor material has at least 7 fused aromatic rings and said polycyclic donor material has at least 8 fused aromatic rings.
 11. An element as defined in claim 9, wherein said acceptor material is a dye or pigment.
 12. An element as defined in claim 9, wherein said acceptor material is a dye selected from the group consisting of pyrylium, thiapyrylium and selenapyrylium dyes.
 13. In a photovoltaic element includinga layer of a compound having the structure: ##STR35## wherein: M is a metal;T¹ and T² are both S or both C, or one of T¹ and T² is N and the other C; X¹ and X² are the same or different, and are each halogen or hydrogen; and Z¹ is the nuclear carbon atoms necessary to form a six-membered unsaturated ring; an organic electron acceptor material capable of forming a rectifying junction with said compound, and electrodes in operative ohmic contact with at least part of said layers; the improvement wherein said acceptor material comprises a compound containing a generally planar, polycyclic nucleus having a surface area of at least about 40 square angstroms and a width of at least about 5 angstroms, and the combined thickness of said layers does not exceed about 0.5 microns, whereby said element has a conversion efficiency of at least about 0.02% when exposed to an AM2 light source.
 14. An element as defined in claim 13, wherein said acceptor material has the structural formula: ##STR36## wherein R⁷ and R⁸ are the same or different and are each hydrogen, alkyl containing from 1 to 5 carbon atoms, phenyl, or quinolyl, and R⁹, R¹⁰, R¹¹ and R¹² are each ═O; or R⁷ and one of R⁹ and R¹⁰, and R⁸ and one of R¹¹ and R¹² together comprise from 7 to 8 nonmetallic atoms necessary to form one or two fused carbocyclic or heterocyclic rings, and the other of R⁹ and R¹⁰ and the other of R¹¹ and R¹² is ═O.
 15. An element as defined in claim 13, wherein said acceptor material has the structural formula: ##STR37##
 16. An element as defined in claim 13, wherein said acceptor material has the structural formula: ##STR38## wherein: J is ##STR39## or N; Q and X' are the same or different and are each O, S or Se;R¹⁷, r¹⁸ and R¹⁹ are the same or different and are each H, alkyl from 1 to about 3 carbon atoms, aryl, cyano or nitro; R¹³, r¹⁴, r¹⁵ and R¹⁶ are the same or different and are each phenyl, or alkyl or alkoxy containing from 1 to about 5 carbon atoms, at least two of R¹³, R¹⁴, R¹⁵ and R¹⁶ being phenyl; m is 1 or 0; and Z⁷⊖ is an anion.
 17. A process of converting incident radiation into electrical power, comprising the steps of(a) exposing to said radiation, a photovoltaic cell comprising a first layer of an organic electron donor material, a second layer of an organic electron acceptor material in contact with said first layer, said layers together producing a combined thickness no greater than about 0.5 microns, at least one of said materials being capable of absorbing light at wavelengths between about 350 and about 1000 nm and both of said materials being capable of forming a rectifying junction between them, and electrodes in operative ohmic contact with at least part of said layers, each of said materials comprising a compound containing a generally planar polycyclic nucleus having a surface area of at least about 40 square angstroms and a width of at least about 5 angstroms, and (b) drawing off power from said cell. 